I think the letters are "ATC", not "ATO" and are standard markings on U.S. built propellers. It stands for "Approved Type Certificate", and sometimes is just abbreviated "TC" followed by a number. I won't be at my other house until tomorrow but I'll see if I can match it to a known design at that time.

Any chance that the "401" is actually "491". If so it would match the Gardner propeller you linked above, and would appear on this page.

I should have some more information about that Type Certificate tomorrow. I suspect that the design applies to a number of different aircraft, mostly WW2 era trainers.